Silver-exchanged zeolites and methods of manufacture therefor

ABSTRACT

The present invention relates generally to zeolites having a silica/alumina ratio of less than or equal to 10 (Si/Al≦10) that are exchanged with Ag +  and thermally treated in such a way to favor adsorption over alternative catalytic and chemically reactive functionalities. The adsorbents of the present invention and the method of producing such adsorbents maximize the working adsorption capacity through π-complexation. Applications for such adsorbents include any process in which contaminants from gas streams can form π-complexes with the Ag in the zeolite, particularly the removal of CO, ethylene, propylene and the like from air and CO/H 2  from air in prepurifier adsorbers in the production of ultra high purity (UHP) N 2 .

CROSS-REFERENCE TO RELATED APPLICATION

The present invention is a divisional of U.S. divisional applicationSer. No. 12/255,018, filed Oct. 21, 2008 now U.S. Pat. No. 7,662,213,which claims priority to U.S. nonprovisional patent application Ser. No.11/170,104, filed Jun. 30, 2005, the entire contents of which areincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to zeolites having asilica/alumina ratio of less than or equal to 10 (Si/Al<10) that areexchanged with Ag⁺ and thermally treated in such a way to favoradsorption over alternative catalytic and chemically reactivefunctionalities. The adsorbents of the present invention and the methodsof producing such adsorbents maximize the working adsorption capacitythrough π-complexation. Applications for such adsorbents include anyprocess in which contaminants from gas streams can form π-complexes withthe Ag in the zeolite, particularly the removal of CO, ethylene,propylene and the like from air and CO/H₂ from air in prepurifieradsorbers in the production of ultra high purity (UHP) N₂.

BACKGROUND OF THE INVENTION

There is a need to remove and/or recover contaminants from industrialprocess gas streams. Separation by adsorption is generally preferredover catalytic and reactive methods since reaction byproducts can beavoided and regeneration/reuse of the adsorbent is possible. There is acertain class of contaminants that are very weakly adsorbed in zeolitescontaining alkali and alkaline earth cations, but the adsorption of suchcontaminants can be greatly enhanced by exchange with Cu, Ag or Au. Thisenhancement arises when the physisorption forces are significantlyaugmented by chemisorption through π-complexation. Molecules of such aclass are small enough to enter the zeolite structure and possess anelectron structure that easily forms π-bonds with the cations in thezeolite, e.g. carbon monoxide (CO), ethylene (C₂H₄), propylene (C₃H₆),etc.

The equilibrium adsorption capacity of CO in AgX and AgY was measured byHuang (J. Catal., v32, 482-491, 1974) as pure component isotherms. About1.0 mmol/g CO is adsorbed on AgX powder at an equilibrium pressure of0.1 torr and 25° C. Similar amounts of N₂ are adsorbed at pressuresabove 100 torr. While this CO equilibrium capacity is attractive, thesedata do not confirm the utility of such materials in real processesdesigned for removal and/or recovery of CO or other contaminants ofinterest. More particularly, the working capacity of CO reflectingcoadsorption of competitive components in the gas stream and theadsorption kinetics in agglomerated zeolites must be considered. Themass transfer front characteristics and the presence of adsorptioninhibitors such as oxidation and reduction agents are also importantconsiderations in practical applications.

One example of these real process effects is the removal of CO from airat normal ambient temperatures. CO and H₂ are present in atmospheric airat concentrations of approximately 0.5 ppm to 10.0 ppm. H₂ may act as areducing agent to Ag⁺, while O₂ (about 21 vol %) acts as an oxidizingagent and is also coadsorbed along with N₂. Thus, H₂ and O₂ can bedeactivating agents with respect to the Ag⁺ cation, while O₂ and N₂compete for adsorption space with the CO. In addition, at lowconcentrations of CO, the CO mass transfer front is diffuse and spreadsover the entire adsorbent bed, i.e. with no apparent equilibrium zone.For these reasons, the equilibrium CO capacity data obtained from purecomponent isotherms is of limited value in predicting the effectivenessof an adsorbent for CO removal from an actual process stream.

In addition, large scale adsorbent processing introduces variables andproblems absent from laboratory or bench-scale processing such that theproduction of large quantities of adsorbent with desirable properties isnot guaranteed. Some of the problems inherent in processing largequantities of solid materials include uniformly mixing and heating thesolid, the distribution of purge flow, purge/solid contact, rapidelimination of water vapor and disposal of waste solutions. Other issuesinclude the costs of dry contaminant-free purge gas, exchange solutionsand deionized wash water and the capital equipment required to handlelarge quantities of solids. Moreover, prior art techniques demonstratehigh variability and lack of consistency in performance of suchmaterials.

Given the numerous types of aluminosilicate zeolites combined withvariations in Si/Al ratio and charge-balancing cations, much literatureon zeolites has been developed. The primary purpose of such technicalliterature is to demonstrate the synthesis of the material and tocharacterize its physical, chemical and adsorptive/catalytic properties.Conditions and practices typical in such teachings are the use of gramquantities of adsorbent processed (often in powder form), use of ionexchange solution(s) and deionized wash water in large excess relativeto the quantity of adsorbent and minimal requirement for disposal ofwaste chemicals. Very slow heating is typically applied in combinationwith large amounts of dry inert purge or evacuation during thermalactivation of the material. Such practices are neither economical norpractical for industrial scale production of the adsorbent or catalyst.

The prior art recognizes a number of applications for Ag-exchangedzeolites, e.g. scavenging of H₂ from vacuum spaces, Ar/O₂/N₂separations, as a bactericide in water purification and separation ofliquid phase aromatic hydrocarbon isomers.

Matsch, et al. (U.S. Pat. No. 3,108,706) describes AgX zeolite for useas a H₂ getter in maintaining the vacuum insulated annulus space ofcryogenic storage containers. In this application, H₂ is removed bychemical reaction with the Ag-exchanged zeolite, i.e. the Ag⁺ and/orexcess Ag₂O contained in the zeolite are reduced by the H₂.

Rosback (U.S. Pat. No. 3,969,276) provides a method of manufacturing anadsorbent comprising ion exchanging either an X or Y zeolite withcations from Group IA, IIA and IB of the Periodic Table. The preferredadsorbent is BaX or BaKX for separating liquid phase mixtures ofhydrocarbons.

Chiang, et al. (U.S. Pat. No. 6,432,170) describe a LiAgX zeolite with asilver exchange level from 20% to 70% for separation of Ar/O₂ gasmixtures. The aim of this invention is to produce an adsorbent withenhanced Ar selectivity over O₂ at a reduced cost (by minimizing thesilver content).

A method for preparing AgA zeolite with adsorptive selectivity of N₂over O₂ in air and Ar over O₂ is disclosed in U.S. Pat. No. 6,572,838 toSebastian, et al. A single step ion exchange process using an aqueoussolution of any silver salt is said to create an adsorbent insensitiveto hydroxylation.

U.S. Pat. No. 4,019,880 to Rabo et al. discloses Ag-exchanged zeoliteswith a high affinity for CO in the presence of water vapor and CO₂.Zeolites preferred by this invention include the high silica zeolites ofthe ZSM-series having 20≦SiO₂/Al₂O₃≦200. Although tolerant to H₂O andCO₂, these zeolites are subject to deactivation by H₂ as the Ag⁺ cationsare reduced by H₂.

Coe, et al. (U.S. Pat. No. 4,544,378) relates to a CaX zeolite for airseparation, attempting to avoid the damaging effects of both zeoliteframework and cation hydrolysis through special attention to the thermalactivation step in processing of the adsorbent. LiAgX zeolites aredisclosed for N₂ adsorption in air separation in U.S. Pat. No. 6,780,806by Yang et al. Preferred formulations include Si/Al=1.0 with Ag<20% ofexchangeable cations. The increased N₂ adsorption capacity is attributedto the supplementary weak chemical bonds formed between N₂ and Agclusters due to π-complexation. It is suggested, however, that fullyexchanged AgX is not favorable for adsorptive separation due to strongadsorption of N₂ at low pressure, resulting in difficulty in desorbingthe N₂ for regeneration of the adsorbent.

Ag-exchanged zeolites created for applications such as gettering of H₂(U.S. Pat. No. 3,108,706) are thermally treated to reduce, react and/oractivate the Ag, resulting in the destruction of part or most of themicropore volume of the zeolite. Thus, catalytic or chemical reactivefunctions are promoted at the expense of adsorption capacity.

With few exceptions, the prior art implicitly assumes that theadsorption characteristics resulting from bench-scale processing of ionexchanged zeolites prepared for gas separations are automaticallyreproducible in large-scale manufacture. Such thinking completelyignores the physical and economic limitations in processingindustrial-scale quantities of zeolites. Ion exchange, drying andthermal activation steps, each involving heat and mass transfer, do notscale linearly from the gram quantities of adsorbents typical oflab-processing to the large kg quantities required for efficientmanufacture. As mentioned above, prior art techniques demonstrate highvariability and lack consistency in performance of such materials.

In addition, the high cost of Ag used in the ion exchange solutions, thedisposal of their waste and the costs of providing dry purge gas cannotbe ignored in large-scale manufacture.

Commonly owned PCT international publication No. WO 03/101587 A1,entitled “Production of High Purity and Ultra-High Purity Gas”,discloses the use of Ag-exchanged zeolites for removing CO in airseparation plant prepurifiers. The entire contents of PCT internationalpublication No. WO 03/101587 are incorporated herein by reference.

While the prior art provides several examples of Ag-exchanged zeolitesfor a variety of separations, the prior art lacks high performanceadsorbents exhibiting consistently high dynamic capacity for gasmolecules receptive to π-complexation such as CO, C₂H₄, C₃H₆ and thelike.

It would thus be desirable to consistently provide adsorbents thatachieve high working capacity as well as perform well in the presence ofhigh concentrations of gases that would normally be strongly coadsorbedby physisorption (N₂, O₂, CO₂, etc.) or in the presence of strongreducing (e.g. H₂) and oxidizing (e.g, O₂) agents.

BRIEF SUMMARY OF THE INVENTION

The present invention provides adsorbents and methods of manufacturetherefor in which the adsorbents demonstrate improved, consistent andreproducible working capacity characteristics. The adsorbents can beused in the presence of high concentrations of gases that wouldtypically be strongly coadsorbed by physisorption (N₂, O₂, CO₂, etc.).The adsorbents of the present invention also have improved performancecharacteristics in the presence of strong reducing (e.g. H₂) andoxidizing (e.g., O₂) agents.

The unique characteristics of the adsorbents of the present inventionare achieved from a combination of factors including: the selection ofzeolites with Si/Al<10, high Ag⁺ exchange (e.g., ≧80%), control of theexcess Ag and other contaminants deposited in the pores of the zeolite,controlling the moisture content to ≦2.0 wt % and maximizing retentionof micropore volume (for example, ≧85%). More specifically, theadsorbents of the present invention are characterized by limitinghydrothermal structural damage (i.e., destruction of micropore volume)to no more than 15%. These properties are largely achieved through theproper combination of processing conditions practical for industrialscale production. The desired processing conditions of the presentinvention avoid practices common in the prior art such as pretreatmentof the zeolite with caustic solution, vacuum activation, rapid heatingand static oven activation.

As mentioned above, one aspect of the adsorbents of the presentinvention are zeolites with Si/Al<10 and at least 80% of exchangeablecations replaced with Ag⁺. The adsorbents are further characterized bylimiting the excess Ag content to no more than 10%, and preferably nomore than 5%, above the fully exchanged charge-balancing Ag cationcapacity of the zeolite. In addition, the final moisture level of thezeolites is no more than 2.0 wt %, and preferably is 1.0 wt % or less.The adsorbents of the present invention exhibit hydrothermal structuraldamage of no greater than 15% and preferably no greater than 10%. Mostpreferably, the hydrothermal structural damage is no greater than 5%.Moreover, the dynamic working capacity or ΔCO of the adsorbents of thepresent invention is at least 0.045 mmol CO/g Ag-zeolite, morepreferably≧ 0.055 mmol/g and most preferably ≧0.065 mmol/g, as measuredby a reference breakthrough test from which a dynamic working capacityof CO is determined.

Another aspect of the present invention is a method of making suchimproved adsorbents in which the following steps are used to produceadsorbents for large scale manufacture. Zeolites with Si/Al<10 areselected and exchanged to replace at least 80% of exchangeable cationswith Ag⁺. The Ag-exchanged zeolites (or Ag-zeolite) are washedsufficiently to eliminate excess Ag, Ag compounds and/or Ag salts orother exchangeable cations or impurities from the pores of the zeolite,with such excess Ag content being limited to no more than 10%, andpreferably no more than 5% above the full charge-balancing Ag cationcapacity of the zeolite. The zeolite is thermally dried and activatedsuch that the exchanged zeolite achieves a final moisture level in theproduct of no more than 2.0 wt % and preferably achieves a finalmoisture level of 1.0 wt % or less. The thermal treatment isaccomplished so as to minimize the hydrothermal structural damage to nogreater than 15%, preferably no greater than 10% and most preferably, nogreater than 5%. As a result, adsorbents can be consistently providedhaving a dynamic working capacity or ΔCO of the Ag-zeolite of at least0.045 mmol CO/g Ag-zeolite. Preferably, the CO adsorptive capacity is0.055 mmol/g, and most preferably ≧0.065 mmol/g, as measured by areference breakthrough test from which a dynamic working capacity of COis determined.

The adsorbent characteristics and methods of production described abovemaximize the working adsorption capacity of the zeolites andconsequently allow for enhanced π-complexation by favoring adsorptionover alternative catalytic and chemically reactive functionalities.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference should be made to the following DetailedDescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates the relationship between CO breakthrough time andMcBain O₂ capacity based on the results of Examples 1-3;

FIG. 2 illustrates the heating and drying rates of the adsorbents fromBatches R_(—)916 and R_(—)922 in Example 4;

FIG. 3 shows the retained O₂ capacity as a function of CO breakthroughtime

FIG. 4 illustrates CO and H₂ breakthrough characteristics of Example 6and indicates the effect of washing upon the mass transfer resistance;

FIG. 5 is a schematic diagram of an exemplary adsorbent arrangement inan adsorbent vessel/bed in accordance with the present invention; and

FIG. 6 is a schematic diagram of a prepurification apparatus suitablefor use in accordance with the present invention.

DETAILED DESCRIPTION

Due to the high cost of Ag-exchanged adsorbents, it is important tomaximize the performance for the intended contaminant removal.Furthermore, such maximized performance can be consistently realizedthrough the specific methods of manufacture provided in the presentinvention.

The present invention is particularly effective in generating highpurity and ultra high purity (UHP) gas streams using Ag-exchangedzeolites through removal and/or recovery of contaminants byπ-complexation. For the purpose of the present invention, “high purity”means ≦100 ppb (parts per billion) contaminant in the purified gasproduct and “ultra high purity” or “UHP” means ≦10 ppb contaminant inthe purified gas product. Limitations of the prior art are thus overcometo fulfill the practical need to produce industrial scale quantities ofAg-exchanged adsorbents exhibiting consistently high performancecharacteristics.

The Ag-exchanged zeolites of the present invention and methods ofproducing such adsorbents in industrial scale quantities recognizes andaddresses certain limitations in the prior art. While not to beconstrued as limiting and while not intending to be bound by any theory,the present invention recognizes that: (1) CO is often removed from gasstreams by catalytic oxidation (forming CO₂ as a byproduct) due to therelative ineffective adsorption resulting from very weak physisorptionof CO in conventional adsorbents; (2) removing trace impurities from agas stream to generate an ultra high purity product has been found to beparticularly sensitive to increased mass transfer resistance due toadsorbent pore blocking by debris left from the ion exchange; (3)structural and chemical changes are affected by the method ofprocessing, particularly in drying and activation, and adsorbents of thesame general type can have quite different adsorption propertiesdepending upon the method of processing; (4) while ion exchange ofzeolites is a convenient and effective means of dispersing metal atomsuniformly, the various possible functionalities of a given type ofzeolite such as catalytic activity, chemical reaction or adsorption mayeach dictate different thermal and/or chemical processing conditions toachieve the desired performance in the intended application; and (5)prior art teachings predominantly address methods of synthesizing ormodifying zeolites in gram quantities rather than tens or hundreds ofkilograms necessary for industrial use.

The present invention is aimed at adsorption, specifically chemisorptiondesigned to promote π-complexation. Catalytic activity can beundesirable, its presence an indicator of adsorption sites converted tocatalyst sites. In addition, catalytic conversion results in productsthat may require further removal to satisfy overall product purityrequirements.

Metal cations and impurities in the zeolite may be transformed byoxidation, reduction or chemical reaction during thermal and/or chemicaltreatment so as to promote or enhance the overall catalytic potential ofthe zeolite. Such treatment may drive the metal atoms from theintracrystalline space and result in collapse of the micropores, therebyleaving a free metal or metal compound well dispersed and exposed on thesurface of the collapsed zeolite. Such a result may significantlypromote catalysis, yet severely retard adsorption.

Although it is known to wash the zeolite after ion exchange to removeexcess salts, specific criteria for effective washing are seldomdiscussed in the prior art, particularly for industrial scaleproduction. Furthermore, little attention has been given to the effectsof macropore blocking in trace contaminant removal. Macropores in thiscontext refer to the conduits for fluid transport (macropores andmesopores) formed within the agglomerated adsorbent particle. Such poresare in contrast to the micropores inside the zeolite crystal. When themacropores become obstructed, there is an increase in mass transferresistance, i.e. slowing the transport of fluid from the bulk stream tothe zeolite crystals. Shorter retention time of the contaminant in theadsorber, reduced working capacity and earlier breakthrough areconsequences of macropore blocking.

As discussed hereinbelow and as shown in Examples 6 and 7, the washingprocess of the present invention minimizes obstruction of macropores ofthe agglomerated adsorbent particle while the drying and calciningprocess minimizes hydrothermal structural damage of the microporezeolite crystal. In the case of trace contaminant removal, breakthroughis measured at the parts-per-billion (ppb) level. H₂ has been found tobe an effective probe molecule for indicating the severity of macroporeblocking.

Ag-exchanged zeolites may exhibit one or more functionalities (e.g.,catalytic, gettering or adsorption) that can be effectively appliedtoward gas phase and/or liquid phase purification and separation.Usually, however, a particular separation application demands a specificfunctionality of the material. Such functionality directly relates tothe amount, location and state of the Ag distributed in the zeolite. Aspecific and desirable functionality is largely promoted through theprocessing of the zeolite. As a result, different separationcharacteristics of the zeolite are the direct result of the method ofprocessing. The finished product may also possess more than a singlefunctionality, often to the detriment of the desired application.

The unique tendency toward clustering of Ag atoms in Ag-exchangedzeolites can leave the zeolite structure more sensitive to hydrothermaldamage. This sensitivity must be managed through careful control of thethermal processing of the zeolite. Such control of the drying andactivation of zeolite particles is more difficult to affect inlarge-scale manufacture. Vacuum thermal activation, commonly employed inlaboratory (small-scale) processing, is less damaging to the zeolite,but is not practical for industrial-scale thermal treatment of zeolites.Without proper attention to the processing conditions, the materialproperties resulting from manufacturing are likely to be quiteinconsistent and different than those obtained using laboratory methods.

The present invention, aimed at adsorption in Ag-exchanged zeolitesthrough highly promoted π-complexation, escapes or minimizes thefollowing limitations of the prior art: hydrothermal damage to the moresensitive Ag-zeolite structure; sensitivity of Ag cations to reductionby components such as H₂; retention of Ag salts, Ag compounds or otherimpurities deposited on the surface and within the macropores of thezeolite; and the inability to economically and consistently scale-upproduction from gram quantities to industrial scale quantities ofadsorbents while maintaining the desired adsorbent characteristicsthrough reproducible methods of manufacture.

The present invention identifies the properties of the Ag-exchangedzeolite adsorbent required to affect high working capacity of adsorbedgas phase components that form weak chemical bonds by π-complexationwith Ag. A combination of factors is important in achieving the desiredperformance on a consistent and reproducible basis. As discussed above,selecting zeolites with high ion exchange capacity (Si/Al<10),exchanging with Ag to ≧80%, minimizing excess Ag and other impurities byproper washing after ion exchange and programmed thermal drying andactivation (in the presence of a dry purge gas) to achieve a moisturecontent of 2.0 wt % or less can be important factors.

While not intending to be limiting, the present invention isparticularly well suited for the removal of trace contaminants (activein π-complexation) where resistance to macropore diffusion andcoadsorption of major gas phase components (e.g. N₂, CO₂, O₂, etc.) mustbe kept to a minimum and where high purity and ultra high purity (UHP)product is desired. Replacing catalytic oxidation with adsorption (forexample, for the removal of CO from a gas stream) also eliminatesproducts of reaction and offers the option to recover the adsorbedcontaminant.

The use of the precious metal Ag in the highly exchanged zeolites of thepresent invention can result in a relatively high cost of adsorbentrelative to conventional commercial adsorbents such as 13X, 5A and thelike. By maximizing the working capacity of the exchanged adsorbent,however, less adsorbent may be required for any particular separation.Achieving such performance of the adsorbent by processing in commercialscale quantities provides an economy of scale that allows for improvedcompetitive advantage through reduced amount of adsorbent and reducedoverall capital cost. The Ag-exchanged adsorbents of the presentinvention are thus economically competitive with other separationmethods by virtue of the high adsorption performance. Such materials arecapable of high purity and ultra high purity separations, while othermethods or materials may not have equivalent attributes.

As mentioned hereinabove, the Ag-exchanged zeolites of the presentinvention are aimed at adsorption of gases by π-complexation. Zeoliteshaving Si/Al<10 are of particular interest because they can be made richin charge balancing monovalent cations. Low Si/Al ratio combined withhigh Ag exchange level result in the greatest number of Ag⁺ cations, andtherefore, the greatest potential for π-bond formation. Faujasites(Types X and Y), Type A, clinoptilolite, mordenite and chabazite areattractive zeolite candidates. While Si/Al=1.0 provides the maximumnumber of cations, such materials are known to be less thermally stableduring activation, e.g. low silica X (LSX), U.S. Pat. No. 4,544,378. Gasspecies that readily interact with Ag to form π-bonds include CO, C₂H₄and C₃H₆ and the like. For purposes of this invention, adsorptionperformance is demonstrated using CO breakthrough tests. Productionmethods are carried out using commercial grade 13XHP as the raw materialfor subsequent exchange with Ag. 13XHP is NaX beads with aSi/Al=1.2-1.25 and is commercially available from UOP, Des Plaines, Ill.USA.

It has been found that the methods of processing the Ag-exchangedzeolite can be important to achieving maximum levels of π-bondinteraction between the adsorbent and the contaminant(s) in the gasstream. Because there may be more than a single combination ofprocessing conditions from which an adsorbent with the desirableproperties can be produced, the success of the manufacturing proceduresis determined on the basis of the adsorbent characteristics andperformance. Such criteria have been developed in terms of microporevolume retention, Ag exchange level, moisture content and CO workingcapacity. The methods for determining these properties are givenhereinbelow.

Maintaining maximum micropore volume within the zeolite is importantwhen the primary application is gas separation by adsorption.Hydrothermal structural damage and loss of zeolite content result in areduction in adsorption capacity. An effective measure of the microporevolume is the amount of oxygen adsorbed at low temperature and pressureusing the McBain O₂ test method (see e.g., Bolton, A. P., “MolecularSieve Zeolites,” in Experimental Methods in Catalytic Research, Vol. II,ed. R. B. Anderson and P. T. Dawson, Academic Press, New York, 1976,incorporated herein by reference.

The zeolite sample is air dried to a free-flowing condition (usuallyjust below H₂O saturation) prior to the McBain test. It is then placedin the McBain apparatus and slowly dehydrated and activated underevacuation overnight, i.e. at a pressure of about 1×10⁻⁴ torr.Activation occurs as the temperature is ramped from ambient to about400° C. in eight hours and then held at this temperature for anadditional eight hours. The sample is then cooled to liquid N₂temperature (77K) and ultra high purity O₂ is introduced and maintainedat a pressure of 70 torr until equilibrium is reached. The amount of O₂adsorbed (wt %) is determined gravimetrically through an accuratemeasurement of the change in length of a calibrated helical spring.

The McBain O₂ test is a more effective and quantitatively superiormethod than conventional X-ray diffraction for determining structuraldamage. Although the test is often used to determine the fractionalzeolite content of a bound zeolite relative to its unbound crystallinepowder analog, it is applied here as a measure of absolute microporevolume represented by wt % O₂ adsorbed. Comparing the micropore volume(i.e., McBain O₂, wt %) before and after drying/activation provides ameans of identifying damage to the zeolite structure incurred during thedrying/activation step. This eliminates the need to compensate forvarying amounts of binder for zeolites of the same structure, as well asdeveloping separate criteria for various structural types havingdifferent characteristic micropore volumes. As a result, a commoncriteria for Ag-exchanged zeolites of the present invention is to limitstructural damage to a maximum of 15% (preferably 10% or less) duringthermal activation (interpreted as % reduction in micropore volume asdetermined by McBain O₂ capacity). Alternatively, 85% or more of themicropore volume (determined after ion exchange and prior to drying) isto be retained after thermal processing.

Ion exchange of various zeolites with Ag may be performed using anynumber of methods as known in the prior art. Some ion exchange examplesare provided in PCT international publication No. WO 03/101587 entitledProduction of High Purity and Ultra-High Purity Gas, the entire contentsof which are incorporated herein by reference. To provide the mosteffective adsorbents for π-complexation, the zeolite should be exchangedwith Ag⁺ as close to 100% of the exchangeable cation capacity aspossible. Any excess Ag beyond this amount is to be minimized asaddressed below. For the examples of the present invention, ion exchangewas performed using the following procedure.

The desired amount of raw material (in the example, 13XHP beads fromUOP) is loaded into a production column and wetted with deionized water.A second column is similarly loaded and serves as a scavenge column tominimize the amount of Ag in the wastewater. A 0.1M to 1.0M AgNO₃solution is circulated slowly through the first column and then into andthrough the scavenge column from which it exits into a storage tank.After completing the exchange of the initial batch, the scavenge columnbecomes the production column. Solutions of varying concentration arereused from the storage tank (always insuring that subsequent exchangeson a given column are performed at increasing AgNO₃ concentration),thereby minimizing the amount of waste solution for disposal. The seriesof continuous exchanges performed at ambient temperature (as describedabove for the production column) are terminated when the AgNO₃concentration in the effluent solution from the production columnreaches the inlet concentration. An elevated temperature exchange isthen initiated by filling the column with a 1.0M to 5.0M fresh solutionof AgNO₃ and the column is heated to 40° C.-80° C. Upon reaching thedesired temperature, the heaters are turned off and the productioncolumn is allowed to cool for about 30 min. The exchange solution isdrained and deionized water is introduced to begin the washing. Thedeionized water is added to cover the zeolite in the column (about0.51H₂O/l solid) and held for a time period ranging from a minimum of10.0 min to 30 min. The wash water is drained or alternatively sent to ascavenge column if the AgNO₃ concentration is high enough for recycle asexchange solution. Production methods for Example 1 included batchwashes repeated 2-3 times resulting in a total volume of wash waterequal to 1.0-1.5 times the volume of the solid adsorbent until the AgNO₃concentration is <0.02M.

The above description for ion exchange is not limiting and other methodsknown in the art are acceptable, e.g. the use of other Ag salts andvarious concentrations of these salts. Limiting the contact time of thezeolite with the exchange solution and the degree and effectiveness ofthe wash after ion exchange were found to be important considerationsbeyond that typically found in the prior art. Calzaferri, et al. (Chem.Soc. Rev., 2003, v32, 29-37) have noted that excess contact time withthe Ag salt solution can even “damage the morphology of the crystals.”For the purpose of this invention, it has been found that excessivecontact time (days or longer) promotes retention of Ag and otherexchangeable cation compounds within the pores of the zeolite particle.High concentrations of exchange solutions may also attack the zeoliteframework over time. Since the base zeolites of this invention generallyfavor Ag over other cations, the exchange of Ag may usually be affectedwithin a few hours or less.

The effectiveness of the washing influences the rate of uptake of theadsorbates, particularly in trace contaminant adsorption aimed atproducing gases of ultra high purity. The prior art often recites ageneral guideline for washing zeolites exchanged with water-solubleions, i.e. “wash sufficiently to remove excess salts of the exchangesolution.” Sometimes it is suggested that the exchanged ion content bemeasured in the wash solution until it is no longer detected. However,such criteria can be qualitative and ignore the possible plugging of themacropores with the salts of the replaced ion and do not adequatelyaccount for the effects of incomplete ion exchange, adsorbent particlesize and slow dissolution from the pores. For bench-scale ion exchange,very large ratios of wash water to solid can be achieved without highcost of the deionized water or waste disposal. For industrial scaleproduction of the materials of this invention, washing efficiencyaffects adsorption performance and cost of the final product. Variationsto the washing procedure described above are noted in the examplesbelow.

In accordance with the present invention, establishing the washingcriteria from adsorption performance to minimize macropore blockage ispreferred over that determined from the measurement of Ag⁺ concentrationin the wash water. While not to be construed as limiting, the totalvolume of deionized water used can be at least 3 times the volume ofsolid adsorbent following its exchange with Ag. This may beapproximately 6 repeated batch washes of the zeolite.

After washing, liquid should be drained from the Ag-exchanged zeolite.The adsorbent should then be air-dried to remove surface water until theadsorbent is free flowing in preparation for activation. An air or inertdry gas purge, at a temperature of 20° C. to about 60° C., may also beapplied to reduce drying time. This pre-drying process can usually becompleted in one to several hours. The solid may be spread out in asingle layer of particles or continuously mixed to allow exposure to thedrying environment for free and rapid evaporation of the surfacemoisture. Removing as much moisture as possible in a reasonable timefacilitates the final activation that follows. Neither shallow nor deepbed drying/activation are recommended for production of kg amounts ofadsorbents typical of this invention.

As discussed herein, activation involves heating the adsorbent to a muchhigher temperature in order to remove most of the remaining moisture andto stabilize the state and location of the Ag. Since spreading largeamounts of adsorbent in a single layer can require large amounts ofprocessing area, some form of solid mixing or conveying can be usedduring the thermal activation process. This solid motion is important toestablish frequent contact with the heated air or inert gas purge inorder to sweep moisture away from the zeolite surface and promotediffusion of water vapor out of the adsorbent pores. Water must bedesorbed while minimizing any hydrothermal damage to the zeolitestructure. The prior art often teaches vacuum desorption during thermalactivation. While quite effective in minimizing loss of micropore volumeduring activation, evacuation is only practical when processing verysmall quantities of adsorbent and is not economically feasible forindustrial scale production as addressed by the present invention. Themost important factors for activation are integral mixing of the solidwith the purge gas, heating of the solid and gas, low dew point of thegas purge and drying time. Successful combinations of these variablesare identified through the characteristics and performance of theprocessed adsorbent.

The activation conditions have been varied in some of the examples belowto demonstrate the importance of these process conditions. Thermaldrying and activation process conditions resulting in minimalhydrothermal damage and moisture content of 2.0 wt % or less forAg-exchanged zeolites include: (1) after air drying at ambienttemperature (moisture content of zeolite at or below saturation), thetemperature of the zeolite is increased at a maximum rate of 1.0° C./min(preferably 0.5° C./min or less) up to 150° C., where the adsorbent isheld for at least 30 min, preferably 1.0 h to 3.0 h; (2) prior toheating the zeolite above 150° C., the moisture content of the zeoliteshould be a maximum of 8.0 wt %, preferably 5.0 wt % or less; (3) thetemperature of the zeolite is then increased at a maximum rate of 2.0°C./min (preferably 1.0° C./min or less) up to 300° C. to 400° C.,preferably to 350° C.; (4) during the above heating steps, a dry air orinert gas purge is to be in direct contact with the well mixed zeolite,continuously flowing at a rate of 1.0 Nm³/h to 2.0 Nm³/h per kg ofdehydrated Ag-exchanged zeolite; and (5) a dew point of the purge gasless than or equal to 10° C., preferably less than −10° C. and mostpreferably less than −30° C.

These drying and activation conditions were applied to the production ofAgX (70 kg to 100 kg batches), continuously mixing in a 0.5 m³ doubleconical rotating mixer. While this combination of conditions was appliedto successfully achieve the desired moisture content and retainedmicropore volume, such process conditions are enabling but not limiting.These processing conditions can serve to guide one skilled in the art todetermine other appropriate processing conditions, i.e. when taken incombination with the CO working capacity, retained micropore volume,moisture content and Ag-exchange level of the processed zeolite. Forexample, a different combination of processing conditions may berequired for processing other Ag-exchanged zeolite types, for smaller orlarger batches of zeolite and for implementing alternate types ofprocess equipment.

The desired characteristics of the Ag-exchanged zeolites of thisinvention are obtained through the efficient and economic production(batch or continuous) of Ag-exchanged zeolites in 50 kg or largerquantities. The methods described herein apply also to smallerquantities, e.g. 10 kg to 50 kg, but economics favor production in therange of 50 kg-100 kg or larger. Each piece of equipment required in theprocessing is to be of sufficient size to contain the quantities statedabove, e.g. ion exchange bed, oven or activation unit each capable ofcontaining at least 50 kg of final product. Batch processing is mostappropriate in this production size range, with a production time of twodays or less per batch desirable and one day per batch most desirable.Much higher production levels (1000 kg-20,000 kg/day) could favor theuse of continuous processing or combination of batch and continuousprocessing using equipment designed and scaled for this purpose.Nevertheless, the methods described herein generally apply to both batchand continuous processing. Highly Ag-exchanged zeolites in accordancewith the present invention represent both a high performance andhigh-value product, and batch processing often provides the mostcost-effective means of production.

The most important characteristics of the Ag-exchanged zeolite thatclosely reflect the processing conditions while simultaneouslyrepresenting the ability of the adsorbent to affect π-complexation withreceptive adsorbates are the retained micropore volume, Ag exchangelevel, moisture content and CO working capacity. The fraction ofretained micropore volume is determined by the McBain O₂ test asdescribed above. The amount of Ag in the agglomerated adsorbent may bedetermined by Inductively Coupled Plasma Atomic Emission Spectroscopy(ICP) or by the Fire Assay method. Except where noted otherwise, the AgAssay method is used and the Ag assay results (reported as wt % Ag)presented herein were performed by Hazen Research, Inc., of Golden,Colo. See also, Bugbee, A Textbook of Fire Assaying, Third Edition, JohnWiley & Sons, New York, 1940 incorporated herein by reference. Themoisture content is determined using a Mettler DL18 Karl FisherTitrator, commercially available from Mettler-Toledo International, Inc.Such methods are known by those skilled in the art.

The general concept and method of a breakthrough test is also well knownto those skilled in the art. For the purpose of this invention, thebreakthrough or working capacity (ΔCO) is determined from an overallmass balance of the CO in the feed and effluent streams at a CObreakthrough concentration of 100 ppb. The dynamic working capacity ofthe CO adsorbate is established here to represent the ability of theadsorbent to remove contaminants by π-complexation while simultaneouslyincorporating coadsorption and kinetic effects inherent in realprocesses. It is expected that the adsorbates (particularly atmosphericgases) will be completely thermally desorbed from the adsorbents inthermal swing adsorption processes (TSA). Under such conditions Equation(1) represents the CO working capacity (ΔCO):

$\begin{matrix}{{\Delta\;{CO}} = {\frac{m_{in}}{w_{s}}{\int_{0}^{t_{b}}{\left( {y_{in} - y_{out}} \right)\ {\mathbb{d}t}}}}} & (1)\end{matrix}$where m_(in) is the molar feed flow into the bed, y_(in) and y_(out) arethe inlet and outlet mole fractions of CO, respectively, w_(s) is themass of adsorbent and t_(b) is the breakthrough time corresponding to apredetermined breakthrough concentration (100 ppb CO). Determined with aCO concentration reduction ratio of 2.0 ppm/0.1 ppm=20, the workingcapacity inherently captures the kinetic effects resulting from masstransfer resistance. For the purpose of the present invention, theprimary component in the feed gas of the breakthrough test is N₂. Sincethe concentration of N₂ in the feed stream is overwhelming compared tothat of CO, the coadsorption effect of CO upon N₂ is negligible.Conversely, the coadsorption of N₂ may have a significant effect uponthe adsorption of CO. The breakthrough test method as described ispreferred for establishing the working capacity for CO since N₂ and O₂coadsorption, H₂ reduction and mass transfer effects are automaticallyincorporated in the resultant CO loading. Thus, the best adsorbent isone that exhibits high CO working capacity (long breakthrough time) inthe presence of such inhibiting factors.

The working capacity of CO was determined in this way from a CObreakthrough test and was used as a reference indicator of the abilityof the adsorbent to form π bonds. The conditions of the test werecarefully selected to critically evaluate adsorbents for the desiredadsorption capability under realistic process conditions. The referencebreakthrough test was performed at 7.9 bara (114.7 psia), 10° C. and aninlet gas flow rate of approximately 21 slpm (78.7 mol/m² s) using anadsorption column length of 5.9 cm. The feed gas included 2.0 ppm CO,3.0 ppm H₂, 79% N₂ and 21% O₂. The corresponding CO partial pressure atwhich the CO working capacity is determined is 2.3×10⁻⁴ psia (0.012mmHg). Details of this test are given in PCT international publicationNo. WO 03/101587, the entire contents of which are hereby incorporatedherein by reference. Any variations to these reference conditions arenoted in the following examples.

EXAMPLE 1

Samples of commercially available AgX zeolites were obtained fromAldrich Chemical Co., Inc. of Milwaukee, Wis. (C_(—)0) and MolecularProducts, Inc. (C*CHEM Division), located in Lafayette, Colo.(C_(—)1-C_(—)5) as shown in Table 1. These materials are advertised asH₂ getters for use in vacuum spaces (e.g. as described in U.S. Pat. No.3,108,706), for catalyzing H₂/O₂ reactions and for removal of halogensfrom gas streams. Received as (10×16) beads, the average diameter ofthese zeolites was between 1.4 mm and 1.5 mm. The samples C_(—)1-C_(—)5represent different production lots, all dried and activated as part oftheir original processing. However, samples C_(—)2, C_(—)3 and C_(—)4were all received with moisture content greater than 2.0 wt %. Becauseof the high Ag wt %, these zeolites are quite expensive, e.g. more than$200/kg. The Ag wt %, moisture content and McBain O₂ were measured usingthe methods referenced above and are provided in Table 1.

TABLE 1 Commercial AgX Zeolites Moisture Ag McBain O₂ t_(b) @100 ΔCOAdsorbent wt % wt % wt % ppb CO h mmol/g C_0 (38,228-0) NA 36.0* 14.85.4 0.052 C_1 (Ag400) 0.5 38.0 13.3 5.5 0.051 C_2 (Ag400) 2.1 37.2 14.94.7 0.044 C_3 (Ag400) 2.3 37.4 14.8 4.0 0.038 C_4 (Ag400) 2.3 38.2 13.63.2 0.031 C_5 (Ag400) 1.2 37.0 4.2 1.2 0.012 *determined by ICP method,all others determined by Ag assay

In sampling a cross-section of only five individual production lots ofAgX representing prior art production methods, it is quite apparent fromthe breakthrough time, ΔCO working capacity and the McBain O₂ results inTable 1 that the quality and consistency of the prior art AgX is highlyvariable. This variation, as much as a factor of four or more, createsan unacceptable burden in designing to a minimum performance toguarantee contracted product purity to the customer. Achieving andmaintaining an overall competitive advantage (as derived from the highperformance of such an expensive adsorbent) requires the production ofthe adsorbent with consistently high CO working capacity.

A sample of each material was packed into a bed 5.9 cm long and driedprior to CO breakthrough testing to reduce moisture content to less than1.0 wt %. Moisture levels greater than 1.5 wt % to 2.0 wt % interferewith the adsorption of CO. This drying was performed using a dry N₂(<−50° C. dew point) purge flow of 13.6 slpm while the temperature wasslowly ramped from ambient to 350° C. and then held at 350° C.overnight. After cooling under the same purge to ambient temperature, aCO breakthrough test was performed at 27° C., all other conditions beingas stated above in the description of the breakthrough test.

It is noted in this Example and the Examples that follow thatirreversible reactions were observed in the first exposure to the feedcomposition (possibly from reactions with impurities or excess Agcompounds resulting from incomplete washing after exchange), so thefirst breakthrough test is discarded. The results of the second andsubsequent breakthrough tests were found to be reproducible and thuswere used to calculate the CO working capacity. CO and other adsorbedimpurities are removed after a breakthrough test by regenerating theadsorbent bed in dry air (<−50° C. dew point) at a temperature varyingfrom 175° C. to 275° C. for 2.0 h and cooling in dry N₂ for 3.0 h, allat 2.0 slpm. The CO working capacity, determined from Equation (1), isgiven in Table 1.

EXAMPLE 2

Several samples of commercially available 13XHP (10×20) zeolite beads(89429-556P-lot no.) were obtained from UOP of Des Plaines, Ill. 13XHPis a synthetic NaX zeolite with a Si/Al ratio of approximately 1.25. TheMcBain O₂ capacity for a pure binderless NaX with the same Si/Al isapproximately 33 wt % to 34 wt %, while the average O₂ capacity of thebound zeolites of Table 2 is 29.65 wt %. The initial step ofheating/evacuation in the McBain O₂ test thoroughly dries the sample andeliminates any effects of moisture upon the O₂ adsorption. The relativedifference in the McBain O₂ (or difference in micropore volume) betweenthe bound 13XHP and the pure NaX powder indicates that the commercialstarting material (13XHP) contains approximately 10 wt % to 12 wt %binder.

The weight fraction of Ag in a fully exchanged AgX can be computed fromthe well-known structure and composition of the pure dehydrated NaXzeolite (Bolton), i.e. starting with NaX and exchanging essentially allof the 85-86 Na⁺/unit cell with Ag⁺. Accounting for the binder contentof the 13XHP, the amount of Ag in a fully exchanged 13XHP zeolite isapproximately 39 wt % (dehydrated basis).

A CO breakthrough test was conducted with one of the 13XHP samples usinga 5.9 cm long packed bed at a feed temperature of 10° C. and otherwiseat the conditions and procedures described above in Example 1. 13XHP(NaX) zeolite has essentially no capacity for CO at low COconcentration, i.e. CO breaks through 13XHP immediately as shown inTable 2.

TABLE 2 Characteristics of Unexchanged 13X Base Zeolite McBain O₂ t_(b)@100 ppb CO Adsorbent Lot No. Wt % h 13XHP 2010004827 30.18 N/A 13XHP2010006043 29.50 N/A 13XHP 2012005651 29.77 N/A 13XHP 2012005900 29.14immediate

The results of Examples 1 and 2 indicate the Ag content of the samplesin Example 1 represents a minimum equivalent of 92.3% Ag exchange, e.g.for sample C_(—)0 in Table 1, the Ag content=(36 wt %/39 wt%)×100=92.3%. Since the AgX zeolites of Example 1 have significantzeolite damage, such level of exchange exists only in the diminishedundamaged zeolite fraction of the adsorbent, i.e. the damaged fractionof the zeolite is characterized by a collapsed micropore structure thatis inaccessible to CO molecules.

Even though the micropore volume should be relatively unaffected by thereplacement of Na⁺ by Ag⁺, the McBain O₂ wt % will be different for theNaX and AgX zeolites due to the difference in adsorbent density. Theadsorbent particle density (as well as the bulk packed density) of theAgX is approximately 1.48 times that of NaX as can readily be determinedfrom the known compositions of both the NaX and AgX adsorbents. Theexpected O₂ capacity for fully exchanged AgX can be estimated from therange of McBain O₂ data for 13XHP in Table 2, i.e. 29.14/1.48=19.7 wt %to 30.18/1.48=20.4. (The sample standard deviation for a McBainreference adsorbent with an average O₂ capacity of 36.66 wt % is 0.17 wt%). Clearly, the results show that all of the prior art productionsamples of AgX of Example 1 suffer more than 25% loss of microporevolume (presumed here to be due to hydrothermal damage). The results ofExamples 1 and 2 demonstrate that a high level of Ag exchange alone doesnot insure high CO working capacity, but that high Ag exchange must beobtained simultaneous with minimum destruction of the zeolite microporevolume.

This example also establishes three important baselines: (1) the averageamount of adsorbed O₂ (representing the micropore volume of the basezeolite) of four production lots of the bound 13XHP adsorbent is 29.65wt %; (2) the amount of Ag in a fully exchanged 13XHP adsorbent is 39 wt%; and (3) the average amount of adsorbed O₂ (representing the microporevolume of fully exchanged AgX) is 20.4 wt %.

EXAMPLE 3

Samples of commercial AgX from a sixth production lot (C_(—)6) wereobtained from Molecular Products, Inc. (C*CHEM division). C_(—)6(received with a moisture content of 1.9 wt %) was produced by the samemethods as samples C_(—)1 through C_(—)5. A second sample (C_(—)7) wasprepared in the laboratory using the same 13XHP base zeolite, ionexchange solutions and procedures as in the production of C_(—)6 upthrough the completion of the ion exchange and washing steps, i.e. priorto any thermal treatment. A portion of this C_(—)7 sample (50-100 g) wasthen air dried at ambient conditions. A standard 5.9 cm bed was thenpacked with this material (about 11.5 g), placed in a tube furnace andpurged at 13.6 slpm with dry N₂ while simultaneously increasing thetemperature (1° C./min) to 150° C. and holding for several hours. SampleC_(—)6 was also placed in a standard 5.9 cm bed. Both C_(—)6 and C_(—)7beds were then activated at 350° C., followed by CO breakthrough testsperformed at 27° C., using the same procedures described in Example 1.The results of these tests are summarized in Table 3. Breakthrough testswere also conducted on sample C_(—)7 with the H₂ concentration in thefeed reduced from the normal 3.0 ppm to about 200 ppb with negligibleeffect upon the CO capacity. The moisture content of both samples afteractivation and prior to breakthrough testing was <1.0 wt %.

TABLE 3 Effect of Drying/Calcination on CO Performance Moisture AgMcBain O₂ t_(b) @100 ΔCO Adsorbent wt % wt % wt % ppb CO h mmol/g C_6(Ag400)    1.9 37.0 13.1 3.5 0.033 C_7 >15* 37.0 18.5 6.4 0.060 moisturecontent after ambient drying, reduced to 0.3 wt % H₂O after labactivation.

The McBain O₂ results of these two samples indicate the amount ofmicropore volume destroyed in sample C_(—)6 due to the production scalecalcination process, i.e. % loss in microporevolume=(1−(13.1/18.5))×100=29.2%, i.e. 70.8% retained O₂ capacity. Therelationship between CO breakthrough time and McBain O₂ capacity,derived from the results of Examples 1, 2 and 3, is shown in FIG. 1. TheMcBain O₂ tests are conducted on the adsorbents “as received,” althougha “gentle activation” is performed as part of the preconditioningassociated with the McBain test. In this example, C_(—)6 represents anadsorbent fully processed using a prior art production method. C_(—)7represents the adsorbent processed through the same ion exchange andwashing process as C_(—)6, but only C_(—)6 was activated by theproduction method. The CO adsorption capacity is strongly affected bythe amount of micropore volume lost to hydrothermal damage. By using therelative loss in O₂ capacity (loss in micropore volume) as a measure ofzeolite damage (rather than an absolute value of O₂ capacity), simplecriteria can be established for the drying and calcination processesthat can be applied to any Ag-exchanged zeolite of this invention, e.g.AgA, AgY, AgX, etc. This is accomplished as demonstrated in Table 3 bymeasuring the O₂ capacity before and after the drying/calcination stepsin the process. This determines the effect of the thermal processingregardless of the type of zeolite, amount of binder or any prior thermaldamage from processing of the base starting material.

EXAMPLE 4

Industrial scale batch production was initiated by first exchanging13XHP (10×20) (Lot no. 2010004827) with Ag and washing with deionizedwater using the procedure described above, i.e. the same procedure usedin commercial production of AgX such as for the materials of Example 1.The amount of calcined AgX produced in each batch varied between 80 kgand 90 kg.

After draining the water from Batch R_(—)906, the zeolite was placed intrays (shallow bed depth of several cm), placed in a vented (but notpurged) oven and the temperature was set at 260° C. The adsorbent wasremoved after about an hour and placed in a 0.5 m³ double conicalrotating mixer. The temperature of the adsorbent in the mixer wascontrolled initially to increase at about 0.25° C./min up to 175° C. andat about 0.6° C./min from 175° C. to 345° C.

Batches R_(—)916 and R_(—)922 were drained and each placed directly inthe mixer prior to calcination. The heating profiles for these batches,shown in FIG. 2, are similar except that the dwell time at 150° C. wasextended from 1.0 h for R_(—)916 to 2.0 h for R_(—)922. The adsorbent isheated indirectly to a final temperature of 370° C. from electricalelements contained within the mixer. The adsorbent does not contact theheating elements. A continuous ambient air purge was supplied to themixer at a rate of about 100 Nm³/h for all three batches. The moistureand amount of Ag and the O₂ capacity were measured at the end of thermalprocessing using the methods already described. CO breakthrough testswere performed at 10° C., otherwise using the conditions and theprocedure described in Example 1.

The shallow bed oven heating without purge for R_(—)906 resulted in asignificant loss in micropore volume. This loss in micropore volume isreflected in both the McBain O₂ result and the CO breakthrough timeshown in Table 4.

Avoiding the static oven and introducing continuous mixing of the solidwith purge while controlling the heating rate resulted in a AgXadsorbent with little or no loss in micropore volume and a superior COworking capacity and breakthrough time. In fact, the CO capacity ofR_(—)916 is from two to eight times higher than that of the commercialsamples of Example 1. Samples of adsorbent were removed from the mixerat different temperatures in the heating cycle. The moisture content ofthese samples was measured. The heating and drying rates of theadsorbents (Batches R_(—)916 and R_(—)922) are shown in FIG. 2. Thismoisture level/temperature combination represents a successful guide todrying and calcining with minimal hydrothermal damage to the zeolite.

The lower Ag wt % of R_(—)922 represents an exchange of about 84% of theexchangeable Na⁺ cations in the zeolite. The resulting CO breakthroughtime is substantially lower, due in part to the reduced amount of Ag⁺cations. The higher apparent O₂ capacity is consistent with the lowerdensity of the incompletely exchanged adsorbent.

TABLE 4 Industrial Scale Batch Production Results Moisture Ag McBain O₂t_(b) ΔCO Adsorbent wt % wt % wt % @100 ppb CO h mmol/g R_906 0.6 37.011.7 3.0 0.030 R_916 0.6 39.9 20.9 9.5 0.101 R_922 0.7 32.6 21.5 4.20.045

EXAMPLE 5

Six batches (each resulting in 79 kg to 91 kg calcined AgX) wereprocessed from 13XHP (Lot No. 2010004827) using the same equipment,conditions and the procedures described in Example 4 for Batch R_(—)916.The average particle size of the finished product was 1.5 mm, while thebatch moisture content varied from 0.85 wt % to 1.57 wt %. The Agcontent or exchange level varied from 93% to 101% of the exchangeablecations (based upon 39 wt % Ag representing 100% Ag exchange). Themoisture content of the ambient air purge used in the drying/calcinationstep was not controlled and varied according to the ambient relativehumidity. The retained amount of O₂ capacity (as determined by measuringthe McBain O₂ capacity before and after drying/calcination) also varied,i.e. the zeolite reached temperatures above 150° C. while retainingexcess moisture resulting in varying levels of hydrothermal damage. CObreakthrough tests were conducted on 5.9 cm long packed beds at a feedtemperature of 10° C. and otherwise at the conditions and proceduresdescribed above in Example 1. The retained O₂ capacity is shown as afunction of CO breakthrough time in FIG. 3. The trend of increasing COcapacity with increasing micropore volume retention is similar to thatshown in FIG. 1.

A drier was installed and the moisture content of the ambient air purgewas controlled to less than −10° C. dew point at an air flow rate to themixer of 135 Nm³/h. Ten batches (each resulting in 74 kg to 105 kgcalcined AgX) were processed from 13XHP (Lot No. 2012005900) using thesame equipment, conditions and the procedures described in Example 4 forBatch R_(—)916 except as noted herein. The average particle size of thefinished product was 1.4 mm, while the batch moisture content variedfrom 0.62 wt % to 0.98 wt %. The Ag content varied from 94% to 106%(based upon 39 wt % Ag representing 100% Ag exchange). For the purposesof this invention, Ag content ≦100% represents the level of Ag⁺exchange, i.e. percent of exchangeable cations in the zeolite replacedby Ag⁺. For Ag content >100%, the Ag amount above 100% represents Ag inexcess of that required for a fully exchanged zeolite, such excessundesirably present in the pores or otherwise deposited on or within thezeolite adsorbent. CO breakthrough tests were conducted as describedabove in this example. The retained O₂ capacity is shown as a functionof CO breakthrough time in FIG. 3.

The introduction of an adequate flow of dry air purge (intimatelycontacting the solid adsorbent) in combination with a controlled heatingschedule clearly achieves high micropore volume (O₂ capacity) retention.There is greater than 90% O₂ capacity retained (<10% loss in microporevolume due to calcination) in all of the ten batches and more than 95%O₂ capacity retained in nine of those ten batches. Nevertheless, therestill exists a large difference between the lowest and highest CObreakthrough times once the loss in micropore volume has been minimizedthrough proper control of the drying\calcination conditions, i.e. fromabout 5.0 to 9.0 hours.

EXAMPLE 6

While particle size, moisture content and measurement error may allcontribute to varying CO breakthrough time and CO capacity, none ofthese factors (within their range of variation in this invention)explain the variation of CO breakthrough times from about 5.0 h to 9.0 hin FIG. 3, i.e. for retained O₂ capacity ≧90%.

A series of three consecutive production batches processed from 13XHP(Lot No. 2010006043), each producing about 75 kg of finished product,resulted in low performance, i.e. CO breakthrough times ranging from 4.5h to 5.1 h. These batches had been partially exchanged with Ag when theion exchange process was interrupted. The partially exchanged adsorbentwas drained of exchange solution and sealed in drums for about a weekbefore being reloaded into the exchange columns to complete the exchangeprocess. The worst performing of these batches (R_(—)956) was used toinvestigate the effect of additional washing upon CO performance. Thisparticular batch had a larger average particle size (1.8 mm) than thetypical production lots (1.4 mm to 1.5 mm) and lost approximately 10% ofits O₂ capacity in thermal activation. Both of these factors (largerparticle size and loss of micropore volume) contribute to lower overallCO working capacity.

A Ag-exchanged sample was extracted from this production batch prior tothermal activation and subjected to additional washing (bench scale)with deionized water (R_(—)956_a). To insure complete washing, thezeolite sample was washed three times with excess water. The totalamount of water from all three washes was approximately 100 times thevolume of zeolite sample. A sample of the fully processed productionbatch was also taken after thermal activation, carefully rehydrated tosaturation and washed similarly (R_(—)956_b), i.e. with excess watervolume approximately 100 times the zeolite sample volume.

After the bench-scale washing, samples R_(—)956_a and R_(—)956_b weredried and activated using the same procedure as that described inExample 3 for sample C_(—)7. CO breakthrough tests were conducted on 5.9cm long packed beds at a feed temperature of 10° C. and otherwise at theconditions and procedures described in Example 1. The results for thesamples subjected to extra washing are compared with those of theregular production lot R_(—)956 in Table 5. Each CO breakthrough time inTable 5 represents the average of two tests, with a maximum variationbetween test pairs of 0.2 h. The extra washing of the fully processedzeolite (R_(—)956_b) resulted in only modest improvement in CO capacity.However, the additional washing provided immediately prior to thermalactivation (R_(—)956_a) resulted in more than double the CO capacity ofthe fully processed zeolite.

TABLE 5 Effect of Additional Washing upon CO Performance: ProductionScale Moisture Ag McBain O₂ t_(b) @100 ΔCO Adsorbent wt % wt % wt % ppbCO h mmol/g R_956 (1) 0.9 37.9 19.3 4.5 0.045 R_956_a (2) NA NA NA 9.20.089 R_956_b (3) NA NA NA 5.5 0.053 (1) full production, sample testeddirectly (2) production ion exchange, sample rewashed and dried inlaboratory prior to testing (3) full production, sample rewashed andredried in laboratory prior to testing

The extensive washing immediately after ion exchange (R_(—)956_a)results in CO capacity near the maximum shown in FIG. 3, while the samelevel of washing performed only after thermal activation (R_(—)956_b)gives only a modest improvement in CO capacity. The CO breakthroughcharacteristics shown in FIG. 4 clearly indicate the effect of washingupon the mass transfer resistance. The two sets of washing experimentsconfirm that only a fraction of the water-soluble material remainssoluble after thermal treatment.

Evidence of the pore blocking mechanism is apparent in the level of H₂activity on the various samples as shown in FIG. 4. The greatest H₂holdup corresponds to the earliest CO breakthrough. An immediatebreakthrough of H₂ to 3.0 ppm represents 0 holdup, while lower H₂concentrations in the effluent represents higher levels of the H₂retained in the adsorbent, i.e. “H₂ holdup”. The varying levels of H₂holdup are believed to be the result of interaction with impurities inthe pores. Excess salt (AgNO₃), free Ag, Ag₂O and other impurities inthe pores react with H₂ (resulting in the observed increased H₂ holdupas evidenced by lower levels of H₂ breakthrough), corresponding to thesimultaneous reduced CO working capacity due to slow mass transfer. Inthis case, H₂ serves as an indicator or probe of blocked pores, althoughthe additional H₂ holdup is not the cause of the poorer CO performance.

Incomplete washing can manifest itself in several other problems such asincomplete drying, low CO diffusion rates and retention of excess Ag orAg compounds that may produce undesirable catalytic reactions. Notwanting to be limited to any particular theory, it is believed that theperformance improvement observed after the additional washing providesvaluable insight toward maximizing the π-complexing ability of theadsorbent. The washing must be adequate to clean out the excess AgNO₃salt deposits in the macropores (and possibly micropores) of the zeolitefrom the ion exchange. The residual salt (excess Ag and thereverse-exchanged Na or other resident cations in the starting zeolite)is generally removed by washing with clean deionized water, although theamount of water required may not be adequately determined by theconcentration of Ag in the waste wash water.

Since the ions must diffuse out of the zeolite structure and particle,diffusion time is dependent upon the size of the zeolite particles.Variations in particle size, amount of wash water, washing time andexchange time result in inconsistent removal of residual salts(incomplete washing) from the exchanged zeolites. The salt-blocked poressubsequently also impede the removal of water from the zeolite duringthermal treatment, resulting in varying levels of residual moisture inthe end product. A large fraction of any water-soluble salt left in thepores after exchange may melt at temperatures reached during calcinationand be dispersed and impregnated as free Ag or Ag₂O on the walls ofpores. The blocked pores also decrease the diffusion rate of CO andinfluence the effective mass transfer rate and mass transfer zone (MTZ)of the dynamic removal process, i.e. reducing the dynamic CO capacity(shorter breakthrough time). It appears that the secondary mechanisminhibiting optimum CO capacity is the blockage of zeolite pores withresidual salts from the exchange process. Thus, proper washing afterexchange is important to maximizing the π-complexing ability of theadsorbent.

EXAMPLE 7

The AgX production lots of Example 5 typically required from two tothree batch washes. Each batch wash used a volume of deionized waterequal to about 50% of the volume of solid zeolite being washed. Washingwas terminated according to the criteria described in the general ionexchange procedure given above, i.e. until the AgNO₃ concentration inthe effluent wash water is <0.02M. Accordingly, the total volume ofdeionized water used in two to three batch washes equaled 1.0 to 1.5times the volume of solid adsorbent, i.e. far less than the excess waterused in Example 6.

In order to determine if reasonable amounts of additional washing inproduction could improve performance, two lots of AgX (each representingabout 75 kg calcined product) were processed from 13XHP (Lot No.2012005900). This material was subjected to six batch washes followingthe ion exchange step. The total volume of deionized water used wasapproximately three times the volume of solid adsorbent. Except for theadditional washing, the adsorbent was processed as described in Example5 using the dry air purge procedure. The average particle size of thefinished product was 1.4 mm. The Ag-exchange level ranged from 99% to100.5% of the exchangeable cations. Approximately 95% of the O₂ capacitywas retained after thermal activation for both production lots.

CO breakthrough tests were conducted on 5.9 cm long packed beds at afeed temperature of 10° C. and otherwise at the conditions andprocedures described above in Example 1. The average CO breakthroughtimes are summarized in Table 6. Comparing with the production data inFIG. 3, these lots using additional washing resulted in near-maximum CObreakthrough time. Thus, establishing the washing criteria from theadsorption performance to minimize macropore blockage is preferred overthat determined from the measurement of Ag⁺ concentration in the washwater.

TABLE 6 Effect of Additional Washing upon CO Performance: ProductionScale McBain O₂ McBain O₂ Moisture Ag post wash post calcine Avg. t_(b)@ ΔCO Adsorbent wt % wt % wt % wt % 100 ppb CO H mmol/g R_1016 0.6 39.221.3 20.2 8.9 0.089 R_1017 0.5 38.6 21.7 20.5 9.1 0.089

EXAMPLE 8

TABLE 7 Summary of AgX Production Campaigns Production No. of t_(b) @100ppb CO ΔCO Campaign Batches h σ_(tb) mmol/g 1 20 6.5 1.5 0.060 2 10 7.40.77 0.069 3 18 7.6 1.0 0.071

Several production campaigns were conducted in which multiple batches ofAgX were produced and the results are shown in Table 7. The firstproduction campaign included twenty batches of AgX, each batch producingfrom 70 to 100 kg AgX. CO breakthrough tests were conducted for each AgXbatch in each production campaign using 5.9 cm long packed beds at afeed temperature of 10° C. and otherwise at the conditions andprocedures described above in Example 1. The average CO performanceresults for each of the first three production campaigns are summarizedin Table 7. The lowest average ΔCO loading and the highest variability(standard deviation determined for breakthrough time, σ_(tb)) occurredin the first production campaign (1). Part of this campaign wasconducted with an ambient air purge prior to the installation of a purgedrier. Post ion-exchange washing conditions, purge flow, calcinationtemperature programming and purge dew point were being investigated andadjusted throughout the first production campaign as reflected inExamples 4-6. Production campaigns (2) and (3) were conducted under muchbetter control of the drying/calcination step using the methodology ofthe present invention. These latter two campaigns included ten andeighteen batches, respectively, each batch producing an amount of AgXwithin the same range given above for the batch of production campaign(1). The resulting ΔCO loading is significantly higher and moreconsistent for production campaigns (2) and (3) (lower variability)compared to the production campaign (1).

The above examples show that the Ag-exchanged adsorbents of the presentinvention represent a considerable improvement in adsorption separationperformance over currently commercial adsorbents. Relative to thereference dynamic CO capacity, the highest performing AgX adsorbents ofthis invention have up to twice the adsorption capacity of the bestcommercial AgX materials and nearly eight times the capacity of theworst performing commercial AgX adsorbents. Such improvements areobtained from the unique combination of the following: (1) selectzeolites with Si/Al<10 and exchange to replace at least 80% of itsexchangeable cations with Ag⁺; (2) wash sufficiently to eliminate excessAg, Ag compounds or salts of impurities or other exchangeable cationsfrom the pores of the zeolite, such excess Ag content limited to no morethan 10%, preferably no more than 5% above the full exchangeablecharge-balancing cation capacity of the zeolite; (3) thermally dry andactivate the exchanged zeolite to achieve a final moisture level in theproduct of no more than 2.0 wt % (preferably 1.0 wt % or less); (4)thermal treatment to be carried out so as to minimize the hydrothermalstructural damage to no greater than 15% (preferably no greater than 10%and most preferably no greater than 5%) as determined by the McBain O₂capacity; (5) the integrated effects of the above processing to resultin a dynamic working capacity of the Ag-zeolite of at least 0.045 mmolCO/g Ag-zeolite (preferably 0.055 mmol/g and most preferably ≧0.065mmol/g) as measured by a reference breakthrough test from which adynamic working capacity of CO is determined.

As discussed above, the present invention provides Ag-exchanged zeolitesto maximize their potential for enhanced adsorption of the appropriatemolecules through the formation of weak chemical π bonds with Ag.Zeolites with Si/Al ratios <10 are preferred because of their highexchange capacity for Ag. Examples of zeolite types having thischaracteristic include X, Y, A, mordenite, clinoptilolite, chabazite,etc. Both natural and synthetic zeolites may be considered.

Because of the diverse structures represented by these zeolites, therewill be a corresponding variation in micropore volume as defined by O₂capacity. The largest absolute micropore volume (and its correspondingzeolite structure) does not in itself define the best base material forexchange with Ag, or ultimately as the best adsorbent forπ-complexation. For base materials of a given structural type that havealready been thermally treated prior to exchange with Ag, however, thosehaving the highest O₂ capacity are preferred.

While several commercial synthetic zeolites are readily available withNa as the charge-balancing cation, zeolites having other cations may beused as the starting zeolite. Typical cations that may be present in thebase material prior to exchange with Ag are Na, Li, K, Mg, Ca, and thelike, either alone or in combination.

The base material (starting zeolite prior to exchange) may be anagglomerate, an extrudate, bead or in granular form with or withoutbinder. The average size of such agglomerates may be from about 0.4 mmto 4.0 mm. The base zeolite may also be a powder at the time of exchangewith Ag with subsequent forming into agglomerates as indicated above orfor dispersal on or into a substrate to create a membrane, monolith orother structural form to be used as a gas separator.

Since weak chemical bonds provide the primary mechanism for adsorption,separation processes that incorporate thermal regeneration are preferredfor desorption of the adsorbed components. However, separation processesutilizing the adsorbents of this invention may include the thermalswing, pressure swing or displacement means (simulated moving bed (SMB))and combinations thereof for regeneration of the adsorbent. Although theadsorbents of this invention are well suited for use at feed gastemperatures in the range of −50° C. to about 50° C. (e.g., the typicaltemperature range in which most feed gas streams are purified), higheror lower temperatures are not excluded depending upon the separation.Likewise, there are no particular limits upon the operating pressure,although pressures in the range of 0.1 atm to 20 atm would be mosttypical.

The adsorbents of this invention are best suited for the removal ofcontaminant gases from gas streams wherein the contaminant gases areparticularly receptive to forming weak chemical bonds with the Ag⁺cation in the zeolite through π-complexation. The most common gases thatwill interact with Ag in this manner are carbon monoxide (CO), ethylene(C₂H₄) and propylene (C₃H₆), although other appropriate molecules willbond similarly.

Both purification and bulk separations may be affected by the adsorbentsof this invention. The removal of CO from air, N₂, H₂, inert gases ormixtures of these gases are examples of purification. These adsorbentsare particularly effective in removing trace contaminants (ppm (partsper million) levels to 0.5 vol % in the feed stream) to the ppb levelsrequired for production of high purity and ultra high purity gases. Thehigh selectivity and working capacity of Ag-exchanged adsorbents makethese materials ideal for adsorbing contaminants from air byπ-complexation when the concentrations in air are 10 ppm, e.g. CO inair. Examples of bulk separations include recovery of CO from synthesisgas and the separation of olefins from paraffins where the concentrationof the most strongly adsorbed component in the feed stream is≧1.0 vol %.Finally, liquid phase separations such as water purification andhydrocarbon solvent recovery are potential applications for theseadsorbents.

The high working capacity of the Ag exchanged zeolites (e.g. ΔCO ≧0.045mmol/g) of this invention facilitate the use of thin adsorbent layers.This is beneficial in limiting the overall cost of separation usingexpensive adsorbents and where space is limited in either smalladsorbers or in retrofitting of existing adsorbers with limited excessspace.

Ag-exchanged zeolites with Si/Al<10 may become partially or fullydeactivated by H₂O in gas phase separations, although this can bereversed by proper thermal activation or drying. In contrast, these sameadsorbents are particularly effective in removing gases forming weakπ-bonds in the presence of other gases that generally are coadsorbedthrough weak to moderately strong physisorption, e.g. N₂, O₂, CO₂, etc.The adsorbents of this invention are also resistant to reduction (e.g.H₂) and oxidation of the Ag⁺ cation.

The present invention can be used in a TSA adsorber and system for theremoval of CO concentration and, optionally the removal of one or moreof: H₂O, CO₂, and H₂ from an incoming feed stream. An exemplary vesseldesign suitable for removal of CO and optionally, other contaminants, isdescribed below with reference to FIG. 5. The arrow as shown in FIG. 5indicates the direction of gas flow through the adsorber bed/vesselduring the purification step of the process. A TSA prepurifier systemincorporating such a vessel is disclosed below with reference to FIG. 6.

Referring again to FIG. 5, vessel 30 is shown. Vessel 30 optionallycontains a first layer of H₂O adsorbent (31) such as alumina, silica gelor molecular sieve or mixtures thereof to remove substantially all ofthe H₂O entering vessel 30. A second layer (32) of CO₂ adsorbent such as13X (NaX) or 5A or mixture of these is optionally used to removesubstantially all of the CO₂. The CO₂ adsorbent layer can also removeany residual water remaining from the H₂O adsorbent layer (31). A thirdlayer (33) of CO adsorbent is placed downstream of the CO₂ adsorbent.(By the term “downstream”, it is meant closer to the effluent or productend of the adsorber vessel.) It will be appreciated by those skilled inthe art that vessel 30 can be used with adsorbent layer 33 alone. Asubstantially H₂O-free and CO₂-free gas stream enters this CO adsorbentlayer. The CO adsorbent layer is formed of a CO adsorbent of the presentinvention. Alternatively, adsorbent layer (33) is a mixture of a COadsorbent of the present invention with another material. Such mixturesare described in copending, commonly owned U.S. application Ser. No.11/170,109, entitled “Adsorbent and Catalyst Mixtures”, filed on evendate, herewith by Ackley. The entire contents of copending, commonlyowned U.S. application Ser. No. 11/170,109, entitled “Adsorbent andCatalyst Mixtures”, filed on even date, herewith by Ackley areincorporated herein by reference.

It will be appreciated that additional layers above layer 33 or belowlayer 31 can be added for adsorption or catalysis as shown in PCTpublication No. 03/101587, entitled “Production of High Purity andUltra-High Purity Gas”.

In accordance with the present invention, existing prepurifiers can beeasily retrofitted using the-adsorbents of the present invention. Anillustrative process is described herein with reference to FIG. 6. Feedair is compressed in compressor 70 and cooled by chilling means 71 priorto entering one of two adsorbers (76 and 77) where at least thecontaminants H₂O, CO₂ and CO are removed from the air. The adsorbers 76and 77 each have the same adsorbent bed configuration, which may, forexample be one as described with reference to FIG. 5 above. The purifiedair exits the adsorber and then enters the air separation unit (ASU)where it is then cryogenically separated into its major components N₂and O₂. In special designs of the ASU, Ar, Kr and Xe may also beseparated and recovered from the air. While one of the beds is adsorbingthe contaminants from air, the other is being regenerated using purgegas. A dry, contaminant-free purge gas may be supplied from the productor waste stream from the ASU or from an independent source to desorb theadsorbed contaminants and thereby regenerate the adsorber and prepare itfor the next adsorption step in the cycle. The purge gas may be N₂, O₂,a mixture of N₂ and O₂, air or any dry inert gas. In the case of thermalswing adsorption (TSA), the purge gas is first heated in heater 82 priorto being passed through the adsorber in a direction countercurrent tothat of the feed flow in the adsorption step. TSA cycles may alsoinclude a pressure swing. When only pressure swing adsorption (PSA) isutilized, there is no heater.

The operation of a typical TSA cycle is now described in reference toFIG. 6 for one adsorber 76. One skilled in the art will appreciate thatthe other adsorber vessel 77 will operate with the same cycle, only outof phase with the first adsorber in such a manner that purified air iscontinuously available to the ASU. The operation of this out-of-phasecycle is indicated with reference to the numbers in parentheses.

Feed air is introduced to compressor 70 where it is pressurized. Theheat of compression is removed in chilling means 71, e.g. a mechanicalchiller or a combination of direct contact after-cooler and evaporativecooler. The pressurized, cool and H₂O-saturated feed stream then entersadsorber 76 (77). Valve 72 (73) is open and valves 74 (75), 78 (79) and80 (81) are closed as the adsorber vessel 76 (77) is pressurized. Oncethe adsorption pressure is reached, valve 78 (79) opens and purifiedproduct is directed to an ASU for cryogenic air separation. When theadsorber 76 (77) has completed the adsorption step, valves 78 (79) and72 (73) are closed and valve 74 (75) is opened to blow down the adsorber76 (77) to a lower pressure, typically near ambient pressure. Oncedepressurized, valve 80 (81) is opened and heated purge gas isintroduced into the product end of the adsorber 76 (77). At some timeduring the purge cycle, the heater is turned off so that the purge gascools the adsorber to near the feed temperature or optionally, the coolpurge is provided directly to the vessel through a bypass.

One of ordinary skill in the art will further appreciate that the abovedescription represents only an example of a typical prepurifier cycle,and there are many variations of such a typical cycle that may be usedwith the present invention, for example as shown in PCT publication No.03/101587.

As used herein, the ΔCO working capacity is determined from a referencebreakthrough test performed at the following test conditions: 7.9 bara(114.7 psia), 10° C. and an inlet gas flow rate of approximately 21 slpm(78.7 mol/m² s) using an adsorption column length of 5.9 cm. The feedgas includes 2.0 ppm CO, 3.0 ppm H₂, 79% N₂ and 21% O₂. The ΔCO workingcapacity is determined from the results of the breakthrough test usingEquation (1) and a CO breakthrough concentration of 100.0 ppb.

As also used herein, the phrase the “Ag content” means the total amountof Ag present in the agglomerated adsorbent relative to the amountrequired for complete replacement of all of the exchangeable cationswith Ag. The Ag⁺ cation capacity (fully exchanged) is first determinedin terms of a wt % Ag for the agglomerated adsorbent of interest. Forexample and while not to be construed as limiting, a fully exchanged AgXusing a 13XHP base has a Ag⁺ cation capacity equivalent to 39 wt % Ag.This equivalent wt % capacity varies with type of zeolite, as well aswith its binder content. The “Ag content” is then determined from themeasured amount of Ag in the final Ag-exchanged adsorbent product asfollows:

${{Ag}\mspace{14mu}{content}} = {\left\lbrack \frac{{wt}\mspace{14mu}\%\mspace{14mu}{Ag}\mspace{14mu}{in}\mspace{14mu}{zeolite}\mspace{14mu}\left( {{by}\mspace{14mu}{fire}\mspace{14mu}{assay}\mspace{14mu}{or}\mspace{14mu}{ICP}} \right)}{\begin{matrix}{{wt}\mspace{14mu}\%\mspace{14mu}{Ag}\mspace{14mu}{corresponding}\mspace{14mu}{to}} \\{{zeolite}\mspace{14mu}{fully}\mspace{14mu}{exchanged}\mspace{14mu}{with}\mspace{14mu}{Ag}}\end{matrix}\mspace{14mu}} \right\rbrack \times 100}$

By the phrase “hydrothermal structural damage” it is meant:

${\%\mspace{14mu}{Hydrothermal}\mspace{14mu}{Damage}} = {\quad{\left\lbrack {1 - \frac{\begin{matrix}{{{McBain}\; O_{2}\mspace{14mu}{wt}\mspace{14mu}\%\mspace{14mu}{of}\mspace{14mu}{Ag}} -} \\{{exchanged}\mspace{14mu}{zeolite}\mspace{14mu}{after}\mspace{14mu}{calcination}}\end{matrix}}{\begin{matrix}{{McBainO}_{2}\mspace{14mu}{wt}\mspace{14mu}\%\mspace{14mu}{after}\mspace{14mu}{Ag}} \\{{exchange}\mspace{14mu}{and}\mspace{14mu}{washing}}\end{matrix}\mspace{14mu}}} \right\rbrack \times 100}}$

It should be appreciated by those skilled in the art that the specificembodiments disclosed above may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

1. A method of making an adsorbent, the method comprising: providing azeolite material having Si/Al<10; replacing at least 80% of exchangeablecations in the zeolite with Ag⁺; washing the exchanged zeolitesufficiently to eliminate excess Ag, Ag compounds and/or salts ofimpurities or other exchangeable cations from the pores of the zeolite;and thermally treating and activating the exchanged zeolite such thatthe exchanged zeolite achieves a final moisture level in the zeolite ofno more than 2.0 wt % and hydrothermal structural damage of no greaterthan 15%, wherein the zeolite has a ΔCO working capacity of at least0.045 mmol CO/g Ag-zeolite, the ΔCO working capacity determined from areference breakthrough test.
 2. The method of claim 1, wherein the Agcontent is between 80-110%.
 3. The method of claim 2, wherein the Agcontent is between 80-105%.
 4. The method of claim 1, wherein the finalmoisture level in the zeolite is no more than 1.0 wt %.
 5. The method ofclaim 1, wherein the adsorbent is a CO adsorbent.
 6. The method of claim1, wherein the ratio of total volume of water to volume of the zeoliteused for the washing step is at least
 3. 7. The method of claim 1,wherein the thermal treatment is accomplished so as to minimize thehydrothermal structural damage to the zeolite to no greater than 10%. 8.The method of claim 7, wherein the thermal treatment is accomplished soas to minimize the hydrothermal structural damage to the zeolite nogreater than 5%.
 9. The method of claim 1, wherein the Ag-exchangedzeolite has a ΔCO working capacity of at least 0.055 mmol CO/gAg-zeolite.
 10. The method of claim 9, wherein the Ag-exchanged zeolitehas a ΔCO working capacity of at least 0.065 mmol CO/g Ag-zeolite.